Hypothesis

Cortical D2/D3 receptor loss in aging disrupts thalamic gating of dopaminergic signals, leading to maladaptive striatal dopamine dynamics that exacerbate cognitive and motivational decline.

Mechanistic Rationale

While striatal D2 receptors decline modestly (≈3‑5 %/decade) and appear buffered against age‑related loss, prefrontal and midfrontal cortical D2/D3 receptors deteriorate faster (6‑16 %/decade) and their loss uniquely predicts working‑memory decline Papenberg et al., 2025. Cortical D2/D3 receptors are densely expressed in layer VI pyramidal neurons that provide top‑down inhibitory input to the thalamic reticular nucleus (TRN) and specific thalamic relay nuclei governing thalamocortical loops to the striatum. We propose that age‑related cortical D2/D3 loss weakens this inhibitory drive, resulting in TRN disinhibition and elevated burst‑mode firing of thalamic neurons. Increased thalamic bursting amplifies noisy, low‑frequency glutamate release onto striatal medium spiny neurons, which in turn drives aberrant phasic dopamine release that fails to encode reward prediction errors. The resulting signal‑to‑noise degradation forces compensatory down‑regulation of striatal D2 receptors and DAT expression, amplifying the observed 5 %/decade D2 loss and 10‑11 %/decade DAT decline Johansson et al., 2025; Troiano et al., 2010.

Exercise‑induced protection may preferentially preserve cortical D2/D3 signaling via BDNF‑mediated synaptogenesis in layer VI, thereby sustaining TRN inhibition and normalizing thalamic burst patterns. This aligns with data showing active older adults exhibit reduced spatial extent of striatal and midbrain D2 loss Dang et al., 2017.

Testable Predictions

1. Imaging‑physiology coupling: In a cohort of adults aged 60‑80, higher cortical D2/D3 binding potential (measured with [^11C]raclopride PET corrected for partial volume) will predict lower thalamic burst index (derived from simultaneous EEG‑fMRI gamma power in the thalamic relay) and greater striatal dopamine release capacity (measured with [^11C]raclopride displacement during a monetary reward task).
2. Directionality: Pharmacologically enhancing cortical D2 signaling (e.g., with a low dose D2‑preferring agonist administered intranasally to target prefrontal cortex) will reduce thalamic burst activity and normalize striatal dopamine responses in older adults with low baseline cortical D2.
3. Exercise intervention: A 6‑month aerobic exercise program will increase cortical D2/D3 binding by ≥8 % (partial‑volume corrected) and concurrently decrease thalamic burst index and increase reward‑evoked striatal dopamine release relative to a stretching control group.

Falsifiability

If cortical D2/D3 binding does not correlate with thalamic burst measures or striatal dopamine release, or if manipulating cortical D2 fails to alter thalamic or striatal dopaminergic signals, the central mechanistic claim is refuted. Similarly, if exercise does not produce the predicted cortical D2 changes despite improving cardiovascular fitness, the hypothesized exercise‑cortical pathway would be unsupported.